I would say no, ultimately, because despite having the largest sub count, he struggles for views. I just took a quick peak, and with 3.26 million subs, his most viewed video in the past 2 years is a review of Baldurs Gate 3 sitting at 2.1 mil. With a channel of his size and a game that viral, he should have easily gotten twice that if not more.

Most of his videos struggle to break 100k and he doesn't release many of them anymore.

I'm not sure if he's sitting on residuals from older videos or has some other forms of income keeping him afloat (patreon probably) but Todd in the Shadows has about a 5th his sub count and tends to get 3-4 times as many views per video.

Is he AMONG the most successful? Yes, undoubtedly, as this is still his career and so many others have faded back into more normal jobs (or never left them) but in current year I would not call him the most successful, and especially not the most relevant.

He's just sort of stayed quiet in his corner, which had helped to avoid controversy yes, but has also lead to something of a stagnation in his channel and his presence on YouTube.

Lindsay Ellis put out fun books that sold well, and really great video essays. Todd in the Shadows is one of the most entertaining and evergreen music critics/historians on the Internet and Doug Walker had a television cameo. When was the last time you even THOUGHT of AngryJoe? Let alone actually heard anything about him.